Scanning probe microscopy (SPM) has been heretofore developed that measures a physical force produced between a probe tip and a sample to image both the topography of the sample surface and the potential on the sample surface. FIG. 3 schematically shows an example of an atomic force microscope relying on Kelvin probe microscopy that is one example of such scanning probe microscopy. A cantilever 1 has electrical conductivity and resilience and is coated with a metal, such as gold. A probe tip 2 is attached to the front end of the cantilever 1. A sample to be investigated is indicated by numeral 3. A piezoelectric scanning device 4 controls the position of the sample 3 on the X-axis (horizontal direction as viewed in FIG. 3), on the Y-axis (direction perpendicular to the plane of the sheet), and on the Z-axis (vertical direction as viewed in FIG. 3). A piezoelectric device 5 for applying vibrations is mounted at the rear end of the cantilever 1 that is supported. The cantilever 1 and the piezoelectric device 5 together form a vibration application means. A light source 6 consists of a laser, for example. The instrument further includes a light detector 7, also known as an optical detector, and a preamplifier 8. An oscillator 9 supplies an output signal having an adjusted amplitude to the piezoelectric device 5 to produce vibrations. A lock-in amplifier or RMS-DC amplifier 10 receives the output signal from the oscillator 9 and selects signals corresponding to amplitude variations synchronized to the output signal from the oscillator 9 to produce a topographic image. The instrument further includes a first error amplifier 11, a filter 12, a z-motion piezoelectric element drive power supply 13 forming a distance control and drive means, a first lock-in amplifier 14 for imaging surface potentials, a second lock-in amplifier 15 for imaging a gradient of capacitance between probe tip 2 and sample 3, an oscillator 16 for supplying a reference signal consisting of an alternating voltage of a given frequency .omega. to the first and second lock-in amplifiers 14 and 15, respectively, the amplitude of the reference signal being adjusted to a desired level, a second error amplifier 17, and an accumulator 18 for applying a potential representative of the sum of the output signal from the oscillator 16 and the output DC voltage Vdc from the second error amplifier 17 to the cantilever 1. The second error amplifier 17 makes a zero adjustment, i.e., produces the output signal Vdc such that the input from the first lock-in amplifier 14 becomes zero. The second error amplifier 17 includes a filter or other element used to feed the DC voltage Vdc back to the cantilever 1.
This atomic force microscope is a noncontact atomic force microscope in which the probe tip 2 and the sample 3 are opposite to each other and are not in contact with each other. Laser light or other light is emitted from the light source 6 and focused onto the rear surface of the cantilever 1. Light reflected from the rear surface impinges on the light detector 7. The light source 6, the cantilever 1, and the light detector 7 together form an optical lever-type detection system for detecting deflections of the cantilever 1. An atomic force exerted between the tip 2 and the sample 3 deflects the cantilever 1, varying the reflection angle. This, in turn, changes the position on the light detector 7 as the light hits the detector 7 spaced from the cantilever 1. The amount of deflection of the cantilever 1 is detected from the change in this position.
In the atomic force microscope constructed in this way, the output signal from the oscillator 9 is supplied to the piezoelectric device 5. Thus, the cantilever 1 is vibrated at a frequency approximately equal to its resonance frequency. Under this condition, if the tip 2 is brought to a position spaced several nanometers from the sample 3, a physical force gradient produced between the tip 2 and the sample 3 deflects the cantilever 1. This varies the output from the light detector 7. The varied output signal is amplified to an appropriate amplitude by the preamplifier 8 and supplied to the lock-in amplifier 10 for producing a topographic image. This lock-in amplifier 10 compares the frequency of the output signal from the light detector 7 with the frequency components contained in the output signal from the oscillator 9 and produces a signal proportional to the amplitude of the common frequency component to the first error amplifier 11. This amplifier 11 maintains the difference between the output from the lock-in amplifier 10 and a certain voltage set according to a reference voltage V, i.e., the deviation from the resonance frequency. The output signal from the first error amplifier 11 is sent to the z-motion piezoelectric element drive power supply 13 via the filter 12. This power supply 13 provides feedback control of the piezoelectric scanning device 4 by producing a z-signal for controlling the z-coordinate of the sample 3 to control the distance between the tip 2 and the sample 3 according to the output signal from the filter 12.
The filter 12 regulates the operation of the feedback circuit described above. The output signal from the filter 12 creates a topographic image of the surface of the sample 3. A signal representing the topographic image is sent to a display unit (not shown). The tip 2 of the sample 3 is scanned in two dimensions in the X- and Y-directions while maintaining constant the distance between the tip 2 and the sample 3. In this way, a topographic image of the surface of the sample 3 is displayed on the display unit.
The output from the light detector 7 is applied via the preamplifier 8 to the first lock-in amplifier 14 for imaging the surface potential of the sample 3 and to the second lock-in amplifier 15. These lock-in amplifiers 14 and 15 are supplied with the reference signal consisting of an alternating voltage of the given frequency .omega. from the oscillator 16. The first lock-in amplifier 14 detects a signal corresponding to the amplitude of the same period (i.e., .omega. component) as the given frequency .omega. of the reference signal. The second lock-in amplifier 15 detects signals corresponding to twice the period (i.e., 2.omega. component) of the frequency .omega. of the reference signal.
The .omega. component detected by the first lock-in amplifier 14 is sent to the second error amplifier 17, which produces the DC voltage Vdc to reduce the .omega. component down to zero, i.e., makes a zero adjustment. The DC output voltage from the second error amplifier 17 is fed to the accumulator 18. This accumulator 18 is also supplied with the AC output signal from the oscillator 16 having the same frequency .omega. as the reference signal, the amplitude of the AC output being adjusted to a given level by an amplitude adjuster incorporated in the oscillator 16. The accumulator 18 produces the sum of the AC voltage of frequency .omega. from the oscillator 16 and the DC voltage Vdc from the second error amplifier 17 to the cantilever 1, thus providing feedback of the voltage.
Application of the AC voltage to the cantilever 1 produces an electrostatic force between the sample 3 and the tip 2 at the front end of the cantilever 1. The sample 3 is at ground potential. The resonance frequency of the cantilever 1 is shifted at the period of the applied AC voltage. The period of this shift is the .omega. component. If the surface potential of the sample 3 and the potential at the front end of the tip 2 are the same, only the 2.omega. component is left. Because the DC voltage Vdc is fed back to the cantilever 1, the surface potential of the sample 3 and the potential at the front end of the tip 2 are kept at the same potential. The DC voltage Vdc from the second error amplifier 17 is the surface potential of the sample 3. A surface potential image of the sample 3 is produced on the display device (not shown) by supplying this DC voltage Vdc to the display device.
The signal of the 2.omega. component detected by the second lock-in amplifier 15 contains information associated with the capacitance between the tip 2 and the sample 3. This signal is imaged on the display device simultaneously with the surface potential.
This example of atomic force microscope uses the so-called Kelvin force probe microscopy (KFM) as a procedure for imaging the surface potential of the sample 3. That is, the electrostatic force is detected directly as a force F, or a deflection of the cantilever 1. The voltage applied to the tip that minimizes the electrostatic force is found. In consequence, the surface potential of the sample 3 with respect to the tip surface is imaged.
The conventional atomic force microscope described already in connection with FIG. 3 and making use of direct detection of the force F must use the cantilever 1 having a small spring constant to ensure detection of the deflections of the cantilever 1. However, reducing the spring constant of the cantilever 1 renders the cantilever 1 more flexible. This creates the danger of the tip 2 at the front end of the cantilever 1 touching the sample 3 and becoming attracted. Therefore, it is impossible to make the distance between the tip 2 and the sample 3 quite small. In consequence, the resolution of the topographic image produced simultaneously with the surface potential image of the sample 3 cannot be improved.
Additionally, the voltage applied across the tip 2 and the sample 3 to detect the surface potential as described above produces an electrostatic force. This electrostatic force increases the distance between the tip 2 and the sample 3. In other words, the tip 2 and the sample 3 are moved away from each other. Consequently, it is not yet possible to produce a topographic image at an atomic resolution by the aforementioned KFM simultaneously with imaging of the surface potential. Accordingly, if the tip 2 and the sample 3 are brought closer to each other by increasing the frequency shift described above, a strong electric field is developed between the tip 2 and the sample 3. This strong field might damage the surface of the sample 3.
On the other hand, in the noncontact atomic force microscopy, a cantilever having a relatively large spring constant has been used as the cantilever 1 in recent years in an ultrahigh vacuum in producing a topographic image of the surface of the sample 3 simultaneously with the surface potential image. This makes it possible to image atoms of the sample 3. Under this condition, if the force F is directly detected as in the prior art technique, the resolution of the surface potential image will deteriorate seriously.